The Beatles were only filmed once while they were actually composing. What song were they working on?

When The Beatles were asked to make a promotional film for Lady Madonna, they decided to have a crack at composing a song while they were being filmed playing. Neil Aspinall later discovered that the snippets used for the promo featured them working on a new song. When he put these pieces of footage together which were used for the Lady Madonna promo, he discovered that it was film of them composing Hey Bulldog, including John and Paul goofing around making animal noises.